Elaborate repair mechanisms evolved to maintain integrity of DNA, and their defects result in a variety of disorders (1-7). Inherited defects in nucleotide excision repair (NER) cause neurodegeneration, as in xeroderma pigmentosum (XP) (1-7), an autosomal recessive disease with defective NER of 254-nm UV-induced (1-7), as well as oxygen free radical-induced (7), DNA lesions. Efficient repair of vital genes in postmitotic neurons is essential to maintain the nervous system, for such neurons cannot be replaced. Defective repair presumably kills XP neurons via lethal accumulation of unrepaired, free radical-induced DNA lesions (5-7).
Neurodegeneration and amyloid deposition occur in familial and sporadic AD and in all patients with trisomy-21 Down syndrome (DS) (4). AD(8) and DS(9) cells have normal survival after exposure to 254-nm UV radiation. AD cells have normal UV-induced unscheduled DNA synthesis in the genome overall (10) and normal removal of UV-induced pyrimidine dimers from transcribing genes (11). But AD and DS cells respond abnormally to the ionizing radiation-type of DNA damage in cell-survival (8, 10-12), chromatid-aberration (13-15), and alkaline-elution (10, 12) assays that are dependent on DNA repair (1, 8, 10-15). However, these assays show abnormalities too small to be diagnostically useful.
G.sub.1 -and G.sub.2 -phase tests which readily distinguish AD from normal cells are described in U.S. application Ser. No. 08/611,330 (35) and also in Parshad et al. (1996) (36). The disclosure of these documents are incorporated by reference herein. In these tests, asynchronously dividing cells are irradiated with fluorescent light. To inhibit DNA repair or processing of radiation-induced DNA damage, either .beta.-cytosine arabinoside (ara-C) for the G.sub.2 -phase test or caffeine for the G.sub.1 -phase test, are added and chromatid breaks arising at the next metaphase (15-17) are quantified. By arresting metaphase cells with colcemid for 1 h starting at either 0.5 h or 14-18 h postirradiation, cells that had been in either the G.sub.2 or G.sub.1 phase of the cell cycle, respectively, when irradiation ended (18) are evaluated. In the G.sub.1 -phase test using fluorescent light, the presence of a significant increase in the amount of chromatid damage in the irradiated cell cultures to which caffeine was added, as compared to the amount of chromatid damage observed in the irradiated cell cultures to which caffeine was not added, aids in the diagnosis of AD in a patient suspected of having AD. In contrast, in the G.sub.1 -phase test using fluorescent light, in normal cells there is no significant difference in the amount of chromatid damage in the irradiated cell cultures in the presence or absence of caffeine. (35, 36).
In the G.sub.2 -phase test using fluorescent light, the absence of a significant increase of at least 15 breaks per 100 metaphase cells in the amount of chromatid damage in the irradiated cell cultures to which ara-C was added, as compared to the amount of chromatid damage observed in the irradiated cell cultures to which ara-C was not added, aids in the diagnosis of AD in the patient suspected of having AD. In contrast, in the G.sub.2 -phase test using fluorescent light, in normal cells there is a significant increase in the amount of chromatid damage in the irradiated cell cultures in the presence of ara-C as compared to the amount of chromatid damage in the absence of ara-C. (35, 36).
The G.sub.2 -phase test is also the subject of U.S. Pat. No. 4,933,274, issued to Sanford, K. K., Parshad, R., and Jones, G. M., entitled "Process for Detecting Genetic Susceptibility to Cancer". This patent describes a G.sub.2 -phase test using either X-rays or fluorescent light to damage fibroblasts or peripheral blood lymphocytes for distinguishing cells of individuals susceptible to cancer from cells of normal individuals. The disclosure of this patent is incorporated by reference herein.
The DNA repair mechanism responsible for the repair of DNA lesions produced by the x-rays in the G.sub.2 -phase test are different from the DNA repair mechanism responsible for the DNA lesions produced in the G.sub.1 -phase with caffeine and the G.sub.2 -phase test with ara-C. The methods described herein provide a means for identifying those compounds with anti-oxidant properties or free radical scavenger properties capable of ameliorating or preventing the free radical damage responsible for the onset of cancer or responsible for amelioration or preventing neurodegenerative diseases, such as Alzheimer disease.